Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: centervolume on January 26, 2026, 05:24:09 pm
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G'day all
This 53 pro chassis PT came in a bit crusty with arcing primaries.
Installing replacement PT which has center tapped 6.3VAC winding.
The original wiring scheme is as follows
from tranny: no center tap, one leg of winding to pin 7/pilot and the other to chassis
sockets: one pin on each 6L6 and 12A*7 soldered to chassis (pins 1,2 on power tubes and pin 9 on pres)
this approach appears to be protocol since the v-front super days and maybe back to the model 26.
I believe the attached drawing may be mislabeled as it doesn't show the 2nd leg of this 6.3VAC heater, (instead showing one leg of the OT secondary as green rather than blue) but the factory wiring has that missing leg soldered to the chassis in accordance with the attached diagram.
Tried floating the CT and replicating the original dressing, it was a lot of racket and clearly unhappy. Isolating the entire winding cleared up the noise to let me verify that was the issue. So my question is - what is the most elegant way to manage this new install?
Looking at subsequent (5E*, 6G* professional series models) heater dressing by Clarence's gang has me wondering if I need to bring both legs down all heater pins...
any help appreciated as always
cv
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The easiest thing to do is ignore the filament center tap, connect one of the greens to chassis, and connect the other green to the pilot lamp. Done.
The improved filament wiring would be to follow one of the later fender schematics with the center tap connected to chassis and the two greens twisted together and strung to all tubes.
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An Humdinger with elevated ground connection will be and improvement (discard the CT and don't use it)
Franco
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Thank you Kagliostro and SEL49
Will look into the humdinger; until then any best practice for isolating the unused leads (heater center tap and the 50v bias winding), other than trim them close and tape them off ?
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For any unused wire: fold back on itself then cover the thing in heatshrink, then tuck away.
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Thank you, roarshock. That is a very neat and safe result.